1. Field of the Invention
The present invention relates to a method of estimating the quantity of boron presenting as an impurity at the bonding interface of a bonded wafer, in terms of the surface density of boron.
2. Description of the Related Art
Bonded wafers are generally classified into two groups depending on the type of bonding: one group being a direct bonding type wafer in which two semiconductor wafers are directly bonded to each other without an insulator between them, and the other group being a so-called SOI (Semiconductor On Insulator) type wafer in which a semiconductor wafer and an insulator wafer, such as oxidized wafers, are bonded together.
Regarding the direct bonding type wafer, when two wafers to be bonded are comprised of mirror-polished, single-crystalline silicon wafers (hereinafter referred to as "Si wafers"), a pair of mirror-polished surfaces of the Si wafers are placed in contact with each other, and after that the superposed Si wafers are heat-treated at a temperature above several hundred degrees to create the strong bonding interface of these two Si wafers. The bonded wafer thus fabricated is equivalent to an epitaxial wafer.
In the fabrication of the SOI type wafer, when two wafers to be bonded are comprised of Si wafers, a silicon dioxide (SiO.sub.2) film is formed on the surface of at least one of two mirror-polished Si wafers. Then, the two Si wafers are superposed, with the SiO.sub.2 film disposed between a pair of mirror-polished surfaces of the Si wafers, and heat-treated at a temperature above several hundred degrees to thermally bond the Si wafers. After the bonding heat treatment, the bonded wafer is thinned by grinding and polishing the surface of the composite insulator wafer including the SiO.sub.2 film. Likewise, a silicon on quartz wafer can be also fabricated.
Bonded wafers are fabricated in a well-controlled clean room. However, since wafers which are cleaned and dried are exposed to air as they are standing by before bonding at room temperature, boron existing in a very small amount in air is adsorbed on wafer surfaces. The thus adsorbed boron may be trapped at the bonding interface in a bonded wafer.
The bonding interface is a region where mirror-polished surfaces of two wafers are directly bonded together in face to face relation. Upon the heat treatment for thermal bonding, the boron impurity trapped at the bonding interface diffuses into the Si single crystal layer, but it is unlikely to diffuse into the SiO.sub.2 insulator layer.
Accordingly, in the case of the above-mentioned bonded wafer equivalent to the epitaxial wafer, the boron impurity diffuses into both of the two Si wafers bonded together. On the other hand, in the case of the SOI wafer composed of a Si wafer forming an activated layer devoid of an oxide film, and a base Si wafer having an oxide film, the boron impurity diffuses into the Si wafer forming the activated layer.
The boron contamination at the bonding interface may reverse the conductivity type of the bonded wafer and deviate the resistivity of the bonded wafer from the designed value. Such a defective bonded wafer, when fabricated into semiconductor devices, deteriorates the stability of performance characteristics of the semiconductor devices and eventually lowers the reliability of the semiconductor devices.
It is, therefore, essential to protect the bonding interface of bonded wafers against contamination. It is also necessary to establish a method of inspecting the presence of contamination and a method of estimating the degree of contamination if the contamination is present. However, when the interface contamination takes place, boron as an impurity diffuses, by the bonding heat treatment, in the direction of the depth of a Si single crystal layer with a certain concentration gradient. Accordingly, an objective estimation of the degree of boron contamination requires a consideration as to in which part of the depth and in which manner the estimation should be done.
It is optimum that the degree of boron contamination, i.e., the quantity of boron at the bonding interface at an initial stage before the bonding heat treatment can be estimated in terms of a surface density. However, no method has been proposed heretofore, which is based on this point of view and enables a measurement of the quantity of boron adsorbed on a wafer surface, e.g., the surface density (atoms/cm.sup.2) of boron.
On the other hand, a change in the concentration (atoms/cm.sup.3) of boron in the direction of the depth of a Si single crystal layer can be measured by the SIMS (Secondary Ion Mass Spectoroscopy). It is, therefore, possible to obtain a boron profile in the direction of the depth by measuring a change in the concentration of boron in the direction of the depth of a Si single crystal layer relative to the bonding interface of a bonded wafer after the bonding heat treatment.
Boron which is present at the bonding interface comes from boron uniformly adsorbed on a pair of wafer surfaces before bonding. The quantity of boron at this initial contamination stage is preferably estimated as a surface density of boron, if possible. However, the above-mentioned boron concentration (atoms/cm.sup.3) change profile in the direction of the depth has shortcomings that (1) only a relative comparison under the same heat treating condition is possible, and (2) a surface density (atoms/cm.sup.2) of boron trapped at the bonding interface at the end of superposition before the bonding heat treatment cannot be determined. In other words, the boron impurity trapped before the bonding heat treatment cannot be estimated or compared by the absolute value of boron surface density (atoms/cm.sup.2).